1. Field of the Invention
The present invention relates to a system which is used to determine the elemental content of materials such as coal and cement, by neutron irradiation. More particularly, the present invention is directed towards an apparatus which divides the flow of material into two paths, subjects the material to neutron bursts, and by utilizing the gamma rays emitted from the interrogated object, provides the ability to calculate the concentration of major and minor chemical elements in the material.
2. Discussion of the Prior Art
Neutron-based elemental on-line analyzers have been used by several industries such as the coal and cement industry over several decades. These analyzers provide a quantitative analysis of the major chemical elements contained in a material. Using neutron producing radioisotopic sources such as californium-252, these on-line analyzers determine the amount of H, N, C, Al, Si, Cl, Ca, S, Fe, etc. contained in the examined materials. Through these measurements, bulk properties such as density, volatile matter, calorific value etc. can be determined using algorithms such as:
Calorific value (BTU/lb)=a(%C)+b(%H)+c(%N)+d(%S)+e(%O)+f(%ash)
However, there are limitations with the apparatus"" used to determine these values.
With the currently available on-line elemental analyzers, the material moves on a conveyor belt or falls continuously through a chute. A neutron emitting radioisotopic source is placed on one side of the chute or conveyor belt, and a set of gamma-ray detectors is placed on the other side or at a position adjacent to the neutron source. The gamma-ray detectors analyze the gamma rays emitted from the coal sample after it is irradiated from the neutrons. These gamma rays act as fingerprints of the elements contained in the material. For example, a 4.43 MeV gamma ray is emitted from carbon, while a 5.42 MeV gamma ray is emitted from sulfur. The gamma-ray signals are transferred to the nuclear electronic modules where they are analyzed, and using the appropriate software, the elemental content of the material in the chute is determined.
The radioisotopic-source based systems, although they measure quite well most of the chemical elements, can be improved by replacing the low energy continuous neutrons emitted from the neutron source with higher energy pulsed neutrons emitted from a pulsed neutron generator. Some of the major advantages from such replacement are:
Direct measurement of carbon (the main element in coal) without interference from other elements.
Measurement of elements such as oxygen and sodium that cannot be measured with a californium-based system.
Major reduction in the radiation hazard during removal and transportation of the source of neutrons.
Reduction of the background in the gamma-ray spectrum.
Ability to utilize a large number of nuclear reactions (fast neutron induced reactions, thermal neutron induced reactions, and neutron activation reactions) which allow improvement of the sensitivity and precision in the measurement of a specific chemical element.
Prior art devices which are disclosed in U.S. Pat. Nos. 5,396,071, 4,841,153, and 4,582,992 utilize isotopic neutron sources and do not allow separation of the measured spectra in spectra produced from fast neutron reactions, thermal neutron reactions, and neutron activation reactions thus limiting their accuracy and calculations of material content. Further prior art disclosed in U.S. Pat. No. 5,162,095 utilizes a neutron generator to examine the flow of bulk material. However, it only uses one pathway for the flow of the bulk material, acquiring at the same location with the same gamma-ray detector spectra from thermal neutron reactions and from neutron activation reactions. In this configuration, all materials (including metal structures, containers, belts, etc.) in the vicinity of the bulk material are also activated. This activation appears as background, obscuring the results from the activation of the bulk material. Furthermore, the prior art devices measure either a thermal neutron spectrum or a neutron activation spectrum, but not both simultaneously. Additionally, in this prior art configuration, approximately 50% of the time either spectrum is not measured.
The present invention is for an elemental on-line material analyzer which utilizes a pulsed neutron generator in combination with a specially designed material flow pathway in order to determine the composition of material flowing through the apparatus. The elements in the material interact with the fast and thermal neutrons produced from the pulsed generator while the material is resident in a first pathway. Spectra of gamma-rays produced from fast neutrons interacting with elements of the material are analyzed and stored separately from spectra produced from thermal neutron reactions. Measurements of neutron activation takes place separately from the above reactions and at a distance from the neutron generator in a second pathway designed particularly for activation reaction analysis.
The present invention is for an elemental on-line material analyzer based on a pulsed neutron generator which analyzes three spectra of data for composition determination. The apparatus utilizes a system which is composed of two pathways for the movement of the material to be analyzed. The main pathway is a chute or alternatively, a conveyor belt that is surrounded by a neutron generator and a system of gamma-ray and neutron detectors. The gamma-ray detectors in the main pathway are triggered by the neutron pulses and they accumulate gamma rays produced from fast neutron and thermal neutron reactions. The neutron detectors monitor the neutron production from the neutron generator and also measure the transmission of neutrons through the bulk of the material. The secondary pathway is a chute or conveyor belt used to measure the gamma rays from elements excited from neutron activation reactions. Elements such as sodium, silicon, aluminum and oxygen are some of the elements more accurately measured through neutron activation reactions. The material flows continuously through the main chute or conveyor, and the gamma ray detectors surrounding the main chute measure continuously the gamma-ray yield on the neutron pulse and off the neutron pulse. The material through the secondary chute flows in batches of a few kilograms each. Each batch is stopped next to the neutron generator where it is irradiated by the pulsed neutrons for a predetermined time. At the end of the irradiation period, the material is quickly moved to another location and a separate gamma-ray detector in the secondary pathway measures for a predetermined time the spectrum produced from the neutron activation. At the end of the measurement period, the material is weighed with an automatic electronic scale, and then is returned to the material flow exiting the main chute or conveyor. The analyzer also contains appropriate high voltage and low voltage power supplies for the production of neutrons, associated data collection electronics, and electrical devices that control the flow of the material samples through the main and the secondary chute. A computer-based data acquisition system receives the appropriate signals generated by the gamma-ray detectors and the various neutron detectors and converts them to digital values. These values are then analyzed by elemental characterization software which determines in real time the amount of the major and minor constituent elements present in the material under interrogation.